The present invention relates to a method and an apparatus for determining the switching state of a transistor having an insulated control electrode, such as, by way of example, a MOSFET or IGBT.
Transistors of this type, which are also referred to as voltage-controlled transistors, turn on and turn off depending on a control voltage present between the drive terminal, the gate terminal in the case of a MOSFET and an IGBT, and one of the load path terminals, the source terminal in the case of a MOSFET and the emitter terminal in the case of an IGBT. When such a drive voltage is applied to the terminal of an initially turned-off transistor, said terminals being externally accessible at a transistor housing, a charging current flows onto the gate electrode in order to charge the latter. In this case, what is important for the switching state of the transistor is the charge that has flowed onto the gate electrode or the internal drive voltage resulting from said charge that has flowed onto the drive electrode, which differs from the externally applied drive voltage owing to parasitic resistances in the lead of the drive electrode and during the switch-on operation owing to the action of the drive electrode as a capacitor. This internal drive voltage can only be tapped off directly at the transistor chip packaged in a housing and is therefore not available for measurements.
Such transistors having an insulated electrode are used in particular as power switches which are constructed in a cellular manner and in which a multiplicity of identically constructed transistors are connected in parallel in order to achieve a high current-carrying capacity and dielectric strength. The data sheets of such components reveal the externally applied gate voltages at which such power switches are switched on, the gate voltages at which they are switched off or the gate voltages at which they are in the transition region between the on state and off state. Precise acquisition of the switch-on and switch-off instants of such power switches is necessary in the case of half-bridges, for example, in which the load paths of two semiconductor power switches are connected in series between two supply potentials and to the common load terminal of which a load is connected. In order to avoid switching losses and in particular in order to avoid a short circuit, it is necessary, in the case of such applications, to ensure that only one of the two power switches in each case turns on. In order to optimize the time sequence of the driving of the two semiconductor power switches, it is necessary to exactly determine in particular the switch-off instants of the power switches in order that one of the power switches is switched on only when the respective other power switch reliable turns off.
In order to determine the switching state of a power switch or in order to determine the switching instants, it is known to use the respective drive signal according to which the drive electrode of the power switch is charged and discharged via a driver circuit. For example, it is known, after a level change in said drive signal, to wait for a fixed time duration until a change in the switching state is indicated by means of a suitable state signal. This waiting time is dimensioned so generously that after the waiting time has elapsed, the power switch reliably turns on or reliably turns off. What is disadvantageous in this case is that the actual instant at which the switching operation is concluded is not determined. Moreover, the generous dimensioning of the waiting time contributes to switching losses.
Furthermore, it is possible to derive the switching state from the externally applied drive voltage. What is disadvantageous in this case is that the internal drive voltage which determines the switching state temporally lags behind said externally applied drive voltage, so that this method is not exact enough to determine the precise instant of switching on and/or switching off. Moreover, the internal drive voltage cannot be determined unambiguously on the basis of the drive voltage applied externally to the component, owing to parasitic effects of the connecting lines and the housing. Furthermore, when evaluating the gate-source voltage in the case of a MOSFET, there is the difficulty that, owing to the known Miller effect, when a drive voltage is applied, the gate-source voltage first of all rises, then remains approximately at a constant level for a specific time duration, before rising further. The switch-on operation takes place during this time period during which the gate-source voltage remains at the constant level, the so-called Miller plateau, so that there is the difficulty of comparing said gate-source voltage with a reference value which lies as little as possible above the plateau for the purpose of determining the switch-on instant, and with a reference value which lies as little as possible below the plateau for the purpose of determining the switch-off state.
The switching state of a power switch can also be determined on the basis of the load path voltage present across the power switch. However, the voltages switched by power switches often lie in the range of from a few tens to a few hundreds of volts. Such voltages cannot be evaluated by means of conventional logic circuits, so that corresponding voltage dividers are necessary, which in turn increase the evaluation costs.